Multifunctional Stimuli-Responsive Supramolecular Materials with Stretching, Coloring, and Self-Healing Properties Functionalized via Host–Guest Interactions

The visualization of changes in the stress and bonding state inside polymeric materials is an attractive function in materials science. In this study, phenolphthalein (PP) was selected to prepare stimuli-responsive coloring materials. PP becomes purple under basic conditions in aqueous solutions; however, PP becomes colorless under basic conditions when it forms a complex with β-cyclodextrin (βCD). To exploit this property of PP, we prepared a color-changing hydrogel (βCD-PP AAm hydrogel) based on acrylamide (AAm) as the main chain and βCD and PP moieties as the side chains. The βCD-PP AAm hydrogel exhibits a color change when heat or a competing molecule is applied at a pH less than 8. This color change was confirmed by ultraviolet–visible (UV–vis) spectroscopy, and the mechanical properties were determined via compression and tensile measurements. The βCD-PP AAm hydrogel also exhibits a rapid, reversible color change upon Joule heating produced by an electric current passing through the gel.